This invention relates to self-tapping screws and methods to connect two or more pieces of thin sheet metal parts to form an assembly.
It is recognized by those skilled in the art that conventional self-tapping screw products, when used to assemble thin sheet metal components, have limited reliability in service due to problems associated with a need to restrain the screw tightening torque to small relative values. This needed restraint on the tightening torque is required to minimize the potential for the anchor to strip during assembly and thereby cause the screw to spin. The anchor material is that part of the joined assembly farthest removed from the underside of the screw head. Stripping and spinning will cause a loss in assembly clamp load and subsequent deterioration of the assembly.
FIG. 1 shows a commonly encountered disadvantage of a self-tapping screw 11 that has a conventional single start thread used to join thin sheet metal components 12 and 16. When the screw anchor material 12, which is that furthest from the head of the screw, is of a width 15 equal to or less than the axial pitch 13 of the screw (defined generally herein as a xe2x80x9cthinxe2x80x9d workpiece), the leading face 14 of the scroll or thread will typically deflect the anchor material 12 such that the material follows the space between adjacent scrolls or threads. This type of anchor material and assembly may not produce the most effective joint clamping load. In addition, inadequate thread mating contact will result.
FIG. 2 shows another potential disadvantage associated with joining thin workpieces using prior art self-tapping screw implementations. The screw 110 maintains a parallel core diameter 111 as close as possible to the underside 112 of the screw head 113. This implementation can cause a reduction in the performance of the assembly. Manufacturing constraints generally create an under-filling of the screw thread crests adjacent to the underside 112 of the screw head 113, thereby creating an unspecified and non-controllable reverse thread taper 114. As such, the thread crests closer to the screw entry point are of a greater diametrical magnitude than the thread crests that are closer to the head. The reverse thread taper 114 has the disadvantage of producing a gap 115 between the internal and external mating threads of the assembly. This gap 115 has the effect of reducing mating thread contact in the essential area of the jointed structure and will result in assembly break down under lower than expected application torque.
It is therefore an object of this invention to provide for a self-tapping screw, and associated joining/fastening method, for connecting a plurality of thin pieces of sheet metal parts that prevents anchor material deflection during the screw assembly and tightening process. A further objective is to ensure that there is minimized any gaps that have previously occurred between the external threads on the screw and the internal threads generated in the anchor material by the self-tapping screw assembly and tightening process.
A self-tapping screw, and associated method, for joining thin work that over-comes the disadvantages of the prior art by providing a tapered root portion of the screw thread root adjacent to the head of the screw that assists in effectively drawing together the thin sheet metal parts of an assembly and additionally developing compressive forces at the interference of the tapered root portion and the anchor material.
The self-tapping screw, in accordance with one embodiment, is additionally provided with scrolls, or screw threads that are of multiple lead threads so that the potential for the screw anchor material to deflect and follow the gaps between adjacent threads on the screw during the assembly and tightening process is eliminated.
In another embodiment, the crest of the threads that are generated on the tapered screw root portion are of the same diametrical magnitude as are the crests of the threads on the body of the screw. In a preferred embodiment, the size diametrical magnitude of the screw thread crests are maintained for a distance that is commensurate withxe2x80x94the start of the screw lead tapered entry portion. The tapered root portion that is adjacent to the underside of the screw head should have an axial length of 2, and preferably no more than 3.5 times that of the axial pitch of the screw thread.
When used to fasten two objects, which are constructed of thin sheet metal work-pieces, the tapered root portion provides resistance to mating thread failure under applied torque by increasing the surface friction that is developed from the compressive forces at the interference with the screw anchor material. The combination of the tapered root portion interference and maintaining a full crest diameter up to the underside of the screw head enhances the ability of the system to resist failure during assembly and service.